Golf ball

ABSTRACT

At all points Pa included in zone “A” away from the central point of the core  4  at a distance of 1 mm or greater and less than 5 mm, the following mathematical expression (I) is satisfied, and at any point Pb included in zone “B” away from the central point of the core  4  at a distance of 5 mm or greater and 10 mm or less, the following mathematical expression (II) is satisfied. 
         Ha 2− Ha 1&lt;5  (I)
 
         Hb 2− Hb 1≧5  (II)
 
     Ha 1  represents the hardness at a point Pa 1  that is located inside the point Pa along the radial direction and away from the point Pa at a distance of 1 mm, and Ha 2  represents the hardness at a point Pa 2  that is located outside the point Pa along the radial direction and away from the point Pa at a distance of 1 mm. Hb 1  represents the hardness at a point Pb 1  that is located inside the point Pb along the radial direction and away from the point Pb at a distance of 1 mm, and Hb 2  represents the hardness at a point Pb 2  that is located outside the point Pb along the radial direction and away from the point Pb at a distance of 1 mm.

This application claims priority on Patent Application No. 2009-297584filed in JAPAN on Dec. 28, 2009. The entire contents of this JapanesePatent Application are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to golf balls. More particularly, thepresent invention relates to multi-piece golf balls having a core, a midlayer and a cover.

2. Description of the Related Art

Top requirement for golf balls by golf players is their flightperformances. The golf players place great importance on flightperformances achieved upon shots with a driver, a long iron and a middleiron. The flight performances correlate with resilience performances ofthe golf ball. Hitting of a golf ball that is excellent in resilienceperformance leads to a high-speed flight, whereby a great flightdistance is attained.

For attaining a great flight distance, an appropriate trajectory heightis required. The trajectory height varies depending on the spin rate andlaunch angle. Golf balls which achieve a high trajectory due to a highspin rate are accompanied by insufficient flight distance. Golf ballswhich achieve a high trajectory due to a great launch angle can attain agreat flight distance. By employing a core having anouter-hard/inner-soft structure, a low spin rate and a great launchangle can be both achieved.

Golf players place great importance also on spin performances of golfballs. A great back spin rate results in small run. For golf players,golf balls which are liable to be spun backwards are apt to be renderedto stop at a target position. Great side spin rate results in easilycurved trajectory of the golf ball. For golf players, golf balls whichare liable to be spun sidewise are apt to allow their trajectory tocurve intentionally. The golf balls that can be easily spun areexcellent in control performances. High-level golf players particularlyplace great importance on control performances upon shots with a shortiron.

In light of achievement of various performances, golf balls having amultilayer structure have been proposed. Japanese Unexamined PatentApplication, Publication No. H10-328326 (equivalent to U.S. Pat. No.6,468,169) discloses a golf ball having an inner sphere, an enclosurelayer, an inner cover and an outer cover. Japanese Unexamined PatentApplication, Publication No. 2001-17575 (equivalent to U.S. Pat. No.6,271,296) discloses a golf ball having a core, an envelope layer, a midlayer and a cover. Japanese Unexamined Patent Application, PublicationNo. 2002-272880 (equivalent to US 2001/0024982) discloses a golf ballhaving a core and a cover. This core is composed of a center and anouter core layer. The cover is composed of an inner cover layer and anouter cover layer. Japanese Unexamined Patent Application, PublicationNo. 2003-205052 (equivalent to US 2003/0166422) discloses a golf ballhaving a center, a mid layer and a cover. Japanese Unexamined PatentApplication, Publication No. 2004-130072 (equivalent to US 2004/0029648)discloses a golf ball having a core and a cover. This core has athree-layer structure.

When a core having an outer-hard/inner-soft structure and having anexcessively large hardness distribution is hit with a driver, greatenergy loss occurs at this core. The energy loss results indeterioration of the resilience performance. When a core having anouter-hard/inner-soft structure and having an excessively large hardnessdistribution is hit with a short iron, a low spin rate is achieved. Thelow spin rate results in deterioration of the control performance.

An object of the present invention is to provide a golf ball that canattain a great flight distance upon hitting with a driver, and that isexcellent in a control performance achieved upon hitting with a shortiron.

SUMMARY OF THE INVENTION

A golf ball according to one aspect of the present invention has a core,a mid layer situated on the external side of the core, and a coversituated on the external side of the mid layer. The proportion of thevolume of the core relative to the volume of the phantom sphere of thegolf ball is no less than 76%. The JIS-C hardness Hc of the cover isless than the JIS-C hardness Ho of the central point of the core. At allpoints Pa included in zone “A” away from the central point of the coreat a distance of 1 mm or greater and less than 5 mm, the followingmathematical expression (I) is satisfied. At any point Pb included inzone “B” away from the central point of the core at a distance of 5 mmor greater and 10 mm or less, the following mathematical expression (II)is satisfied.

Ha2−Ha1<5  (I)

Hb2−Hb1≧5  (II)

In the above mathematical expression (I), Ha1 represents a JIS-Chardness at a point Pa1 that is located inside the point Pa along theradial direction and away from the point Pa at a distance of 1 mm, andHa2 represents a JIS-C hardness at a point Pa2 that is located outsidethe point Pa along the radial direction and away from the point Pa at adistance of 1 mm. In the above mathematical expression (II), Hb1represents a JIS-C hardness at a point Pb1 that is located inside thepoint Pb along the radial direction and away from the point Pb at adistance of 1 mm, and Hb2 represents a JIS-C hardness at a point Pb2that is located outside the point Pb along the radial direction and awayfrom the point Pb at a distance of 1 mm.

In the golf ball according to the present invention, the core has anappropriate hardness distribution. This core is accompanied by lessenergy loss upon hitting with a driver. According to this golf ball, agreat flight distance is attained upon hitting with a driver. This golfball is excellent in a control performance achieved upon hitting with ashort iron.

Preferably, the JIS-C hardness Hc of the cover is no greater than 65.The cover has a thickness of preferably no greater than 0.8 mm.

Preferably, the JIS-C hardness Hm of the mid layer is no less than 90.The mid layer has a thickness of preferably no greater than 1.5 mm.

Preferably, a principal component of the base material of the cover is athermoplastic polyurethane. The polyol component of this thermoplasticpolyurethane is a polytetramethylene ether glycol having a numberaverage molecular weight of no greater than 1,500.

Preferably, the difference between the JIS-C hardness He of the surfaceof the core and the hardness Hb2 is no less than 10. Preferably, thedifference between the hardness He and the hardness Ho is no greaterthan 40.

Preferably, the difference between the hardness Ho and the hardness Hcis 3 or greater and 15 or less. preferably, the hardness Ho is 40 orgreater and 80 or less. Preferably, the hardness He is 75 or greater and95 or less. Preferably, the hardness Hm is greater than the hardness He.

The core may have a center and an envelope layer situated on theexternal side of the center. The center has a diameter of preferably 10mm or greater and 20 mm or less. The envelope layer has a thickness ofpreferably 8 mm or greater and 18 mm or less. Preferably, the differencebetween the hardness He and the JIS-C hardness Hi of the innermost pointof the envelope layer is 10 or greater and 25 or less. Preferably, thehardness He is greater than the JIS-C hardness of the surface of thecenter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a partially cut off cross-sectional view illustrating agolf ball according to one embodiment of the present invention;

FIG. 2 shows a graph illustrating a hardness distribution of the core ofa golf ball according to Example 1 of the present invention;

FIG. 3 shows a graph illustrating a hardness distribution of the core ofa golf ball according to Example 2 of the present invention;

FIG. 4 shows a graph illustrating a hardness distribution of the core ofa golf ball according to Example 3 of the present invention;

FIG. 5 shows a graph illustrating a hardness distribution of the core ofa golf ball according to Example 4 of the present invention;

FIG. 6 shows a graph illustrating a hardness distribution of the coresof golf balls according to Examples 5 to 7 of the present invention andComparative Example 1;

FIG. 7 shows a graph illustrating a hardness distribution of the core ofa golf ball according to Comparative Example 2;

FIG. 8 shows a graph illustrating a hardness distribution of the core ofa golf ball according to Comparative Example 3; and

FIG. 9 shows a graph illustrating a hardness distribution of the core ofa golf ball according to Comparative Example 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be described in detail accordingto the preferred embodiments with appropriate references to theaccompanying drawing.

A golf ball 2 shown in FIG. 1 has a spherical core 4, a mid layer 6situated on the external side of the core 4, and a cover 8 situated onthe external side of the mid layer 6. The core 4 has a spherical center10, and an envelope layer 12 situated on the external side of the center10. A large number of dimples 14 are formed on the surface of the cover8. Of the surface of the golf ball 2, a part other than the dimples 14is land 16. This golf ball 2 has a paint layer and a mark layer on theexternal side of the cover 8 although these layers are not shown in theFigure.

This golf ball 2 has a diameter of from 40 mm to 45 mm. From thestandpoint of conformity to a rule defined by the United States GolfAssociation (USGA), the diameter is preferably no less than 42.67 mm. Inlight of suppression of the air resistance, the diameter is preferablyno greater than 44 mm, and more preferably no greater than 42.80 mm. Theweight of this golf ball 2 is 40 g or greater and 50 g or less. In lightof attainment of great inertia, the weight is preferably no less than 44g, and more preferably no less than 45.00 g. From the standpoint ofconformity to a rule defined by the USGA, the weight is preferably nogreater than 45.93 g.

Preferably, the center 10 is obtained through crosslinking of a rubbercomposition. Illustrative examples of preferable base rubber includepolybutadienes, polyisoprenes, styrene-butadiene copolymers,ethylene-propylene-diene copolymers and natural rubbers. In light of theresilience performance, polybutadienes are preferred. When other rubberis used in combination with a polybutadiene, it is preferred that thepolybutadiene is included as a principal component. Specifically, thepercentage of the amount of the polybutadiene relative to the totalamount of the base rubber is preferably no less than 50% by weight, andmore preferably no less than 80% by weight. The percentage of cis-1,4bonds in the polybutadiene is preferably no less than 40%, and morepreferably no less than 80%.

The rubber composition for use in the center 10 contains aco-crosslinking agent. The co-crosslinking agent serves in achieving ahigh resilience of the center 10. Preferable examples of theco-crosslinking agent in light of the resilience performance includemonovalent or bivalent metal salts of an α,β-unsaturated carboxylic acidhaving 2 to 8 carbon atoms. Specific examples of the preferableco-crosslinking agent include zinc acrylate, magnesium acrylate, zincmethacrylate and magnesium methacrylate. In light of the resilienceperformance, zinc acrylate and zinc methacrylate are particularlypreferred.

In light of the resilience performance of the golf ball 2, the amount ofthe co-crosslinking agent is preferably no less than 10 parts by weight,and more preferably no less than 15 parts by weight relative to 100parts by weight of the base rubber. In light of soft feel at impact, theamount of the co-crosslinking agent is preferably no greater than 50parts by weight, and more preferably no greater than 45 parts by weightrelative to 100 parts by weight of the base rubber.

Preferably, the rubber composition for use in the center 10 includes anorganic peroxide together with the co-crosslinking agent. The organicperoxide serves as a crosslinking initiator. The organic peroxide isresponsible for the resilience performance of the golf ball 2. Examplesof suitable organic peroxide include dicumyl peroxide,1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane,2,5-dimethyl-2,5-di(t-butylperoxy)hexane and di-t-butyl peroxide. Inlight of versatility, dicumyl peroxide is preferred.

In light of the resilience performance of the golf ball 2, the amount ofthe organic peroxide is preferably no less than 0.1 part by weight, morepreferably no less than 0.3 part by weight, and particularly preferablyno less than 0.5 part by weight relative to 100 parts by weight of thebase rubber. In light of soft feel at impact, the amount of the organicperoxide is preferably no greater than 3.0 parts by weight, morepreferably no greater than 2.8 parts by weight, and particularlypreferably no greater than 2.5 parts by weight relative to 100 parts byweight of the base rubber.

Preferably, the rubber composition for use in the center 10 contains anorganic sulfur compound. Illustrative examples of preferable organicsulfur compound include mono-substituted forms such as diphenyldisulfide, bis(4-chlorophenyl) disulfide, bis(3-chlorophenyl) disulfide,bis(4-bromophenyl) disulfide, bis(3-bromophenyl) disulfide,bis(4-fluorophenyl) disulfide, bis(4-iodophenyl) disulfide andbis(4-cyanophenyl) disulfide; di-substituted forms such asbis(2,5-dichlorophenyl) disulfide, bis(3,5-dichlorophenyl) disulfide,bis(2,6-dichlorophenyl) disulfide, bis(2,5-dibromophenyl) disulfide,bis(3,5-dibromophenyl) disulfide, bis(2-chloro-5-bromophenyl) disulfideand bis(2-cyano-5-bromophenyl) disulfide; tri-substituted forms such asbis(2,4,6-trichlorophenyl) disulfide andbis(2-cyano-4-chloro-6-bromophenyl) disulfide; tetra-substituted formssuch as bis(2,3,5,6-tetrachlorophenyl) disulfide; and penta-substitutedforms such as bis(2,3,4,5,6-pentachlorophenyl)disulfide andbis(2,3,4,5,6-pentabromophenyl)disulfide. The organic sulfur compound isresponsible for the resilience performance. Particularly preferredorganic sulfur compounds are diphenyl disulfide, andbis(pentabromophenyl) disulfide.

In light of the resilience performance of the golf ball 2, the amount ofthe organic sulfur compound is preferably no less than 0.1 part byweight, and more preferably no less than 0.2 part by weight relative to100 parts by weight of the base rubber. In light of soft feel at impact,the amount of the organic sulfur compound is preferably no greater than1.5 parts by weight, more preferably no greater than 1.0 part by weight,and particularly preferably no greater than 0.8 part by weight relativeto 100 parts by weight of the base rubber.

Into the center 10 may be blended a filler for the purpose of adjustingthe specific gravity and the like. Illustrative examples of suitablefiller include zinc oxide, barium sulfate, calcium carbonate andmagnesium carbonate. The amount of the filler is determined ad libitumso that the intended specific gravity of the center 10 can beaccomplished. Particularly preferable filler is zinc oxide. Zinc oxideserves not only to adjust the specific gravity but also as acrosslinking activator.

An anti-aging agent, a coloring agent, a plasticizer, a dispersant,sulfur, a vulcanization accelerator and the like may be added to therubber composition for use in the center 10 as needed. In this rubbercomposition may be also dispersed crosslinked rubber powders orsynthetic resin powders.

In light of the resilience performance, the central hardness Ho of thecenter 10 is preferably no less than 40, more preferably no less than45, and particularly preferably no less than 50. In light of suppressionof the spin, the central hardness H1 is preferably no greater than 80,more preferably no greater than 75, and particularly preferably nogreater than 70. The central hardness Ho is measured by pushing a JIS-Ctype hardness scale on a central point of a section of a hemispherewhich had been obtained by cutting the center 10. For the measurement,an automated rubber hardness tester (“P1”, trade name, available fromKobunshi Keiki Co., Ltd.) equipped with this hardness scale is used.

The hardness of this center 10 gradually increases from the centralpoint toward the surface. The surface hardness of the center 10 isgreater than the central hardness Ho.

The center 10 has a diameter of 10 mm or greater and 20 mm or less. Bythe center 10 having a diameter of no less than 10 mm, excellent feel atimpact can be achieved. In this respect, the diameter is more preferablyno less than 12 mm, and particularly preferably no less than 13 mm. Thecenter 10 having a diameter of no greater than 20 mm enables theenvelope layer 12 having a sufficiently great thickness can be formed.In this respect, the diameter is more preferably no greater than 18 mm,and particularly preferably no greater than 17 mm.

The envelope layer 12 is obtained through crosslinking of a rubbercomposition. Illustrative examples of preferable base rubber includepolybutadienes, polyisoprenes, styrene-butadiene copolymers,ethylene-propylene-diene copolymers and natural rubbers. In light of theresilience performance, polybutadienes are preferred. When other rubberis used in combination with a polybutadiene, it is preferred that thepolybutadiene is included as a principal component. Specifically, thepercentage of the amount of the polybutadiene relative to the totalamount of the base rubber is preferably no less than 50% by weight, andmore preferably no less than 80% by weight. The percentage of cis-1,4bonds in the polybutadiene is preferably no less than 40%, and morepreferably no less than 80%.

A co-crosslinking agent is preferably used in crosslinking the envelopelayer 12. Preferable examples of the co-crosslinking agent in light ofthe resilience performance include monovalent or bivalent metal salts ofan α,β-unsaturated carboxylic acid having 2 to 8 carbon atoms. Specificexamples of the preferable co-crosslinking agent include zinc acrylate,magnesium acrylate, zinc methacrylate and magnesium methacrylate. Inlight of the resilience performance, zinc acrylate and zinc methacrylateare particularly preferred.

In light of the resilience performance of the golf ball 2, the amount ofthe co-crosslinking agent is preferably no less than 20 parts by weight,more preferably no less than 25 parts by weight, and particularlypreferably no less than 30 parts by weight relative to 100 parts byweight of the base rubber. In light of soft feel at impact, the amountof the co-crosslinking agent is preferably no greater than 60 parts byweight, more preferably no greater than 55 parts by weight, andparticularly preferably no greater than 50 parts by weight relative to100 parts by weight of the base rubber.

Preferably, the rubber composition for use in the envelope layer 12includes an organic peroxide together with the co-crosslinking agent.The organic peroxide serves as a crosslinking initiator. The organicperoxide is responsible for the resilience performance of the golf ball2. Examples of suitable organic peroxide include dicumyl peroxide,1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane,2,5-dimethyl-2,5-di(t-butylperoxy)hexane and di-t-butyl peroxide. Inlight of versatility, dicumyl peroxide is preferred.

In light of the resilience performance of the golf ball 2, the amount ofthe organic peroxide is preferably no less than 0.1 part by weight, morepreferably no less than 0.3 part by weight, and particularly preferablyno less than 0.5 part by weight relative to 100 parts by weight of thebase rubber. In light of soft feel at impact, the amount of the organicperoxide is preferably no greater than 3.0 parts by weight, morepreferably no greater than 2.8 parts by weight, and particularlypreferably no greater than 2.5 parts by weight relative to 100 parts byweight of the base rubber.

Preferably, the rubber composition for use in the envelope layer 12contains an organic sulfur compound. The organic sulfur compounddescribed above in connection with the center 10 can be used for theenvelope layer 12. In light of the resilience performance of the golfball 2, the amount of the organic sulfur compound is preferably no lessthan 0.1 part by weight, and more preferably no less than 0.2 part byweight relative to 100 parts by weight of the base rubber. In light ofsoft feel at impact, the amount of the organic sulfur compound ispreferably no greater than 1.5 parts by weight, more preferably nogreater than 1.0 part by weight, and particularly preferably no greaterthan 0.8 part by weight relative to 100 parts by weight of the baserubber.

Into the envelope layer 12 may be blended a filler for the purpose ofadjusting the specific gravity and the like. Illustrative examples ofsuitable filler include zinc oxide, barium sulfate, calcium carbonateand magnesium carbonate. Powders constituted with a highly dense metalmay be also blended as the filler. Specific examples of the highly densemetal include tungsten and molybdenum. The amount of the filler isdetermined ad libitum so that the intended specific gravity of theenvelope layer 12 can be accomplished. Particularly preferable filler iszinc oxide. Zinc oxide serves not only to adjust the specific gravitybut also as a crosslinking activator. Various kinds of additives such assulfur, an anti-aging agent, a coloring agent, a plasticizer, adispersant and the like may be blended in an adequate amount in theenvelope layer as needed. Into the envelope layer 12 may be also blendedcrosslinked rubber powders or synthetic resin powders.

In the molding of the envelope layer 12, the center 10 is covered by twopieces of uncrosslinked or partially crosslinked half shell. The halfshells are compressed and heated. The heating causes a crosslinkingreaction, thereby completing an envelope layer 12. The crosslinkingtemperature is usually 140° C. or higher and 180° C. or lower. Thecrosslinking time period of the envelope layer 12 is usually 10 minutesor longer and 60 minutes or shorter.

In this envelope layer 12, the hardness gradually increases from theinnermost point toward the surface. In light of the resilienceperformance, the hardness He of the surface of the envelope layer 12(i.e., the surface of the core 4) is preferably no less than 75, morepreferably no less than 80, and particularly preferably no less than 85.In light of the feel at impact, the hardness He is preferably no greaterthan 95, more preferably no greater than 93, and particularly preferablyno greater than 92. The hardness He is measured by pushing a JIS-C typehardness scale on the surface of the core 4. For the measurement, anautomated rubber hardness tester (“P1”, trade name, available fromKobunshi Keiki Co., Ltd.) equipped with this hardness scale is used.

In light of suppression of the spin, the difference (He−Hi) between thesurface hardness He of the envelope layer 12 and the hardness Hi of theinnermost point of the envelope layer 12 is preferably no less than 10,more preferably no less than 12, and particularly preferably no lessthan 15. In light of ease in manufacture and durability, the difference(He−Hi) is preferably no greater than 25.

The hardness Hi is measured on a hemisphere obtained by cutting the core4. By pushing a JIS-C type hardness scale on a section of thehemisphere, the hardness Hi is measured. The hardness scale is pushed ona region sandwiched between a first circle and a second circle. Thefirst circle corresponds to a boundary between the center and theenvelope layer 12. The second circle is concentric with the first circleand has a radius greater than the first circle by 1 mm. For themeasurement, an automated rubber hardness tester (“P1”, trade name,available from Kobunshi Keiki Co., Ltd.) equipped with this hardnessscale is used.

The envelope layer 12 has a thickness of preferably 8 mm or greater and18 mm or less. The envelope layer 12 having a thickness of no less than8 mm can suppress the spin. In this respect, the thickness is morepreferably no less than 9 mm, and particularly preferably no less than10 mm. The envelope layer 12 having a thickness of no greater than 18 mmenables the center 10 having a large diameter to be formed. The center10 having a large diameter can suppress the spin. In this respect, thethickness is more preferably no greater than 16 mm, and particularlypreferably no greater than 15 mm.

In light of suppression of the spin, the difference (He−Ho) between thesurface hardness He of the core 4 and the central hardness Ho of thecenter 10 is preferably no less than 20, and particularly preferably noless than 25. In light of the resilience performance of the core 4, thedifference (He−Ho) is preferably no greater than 40, and particularlypreferably no greater than 35.

Herein, a zone away from the central point of the core 4 at a distanceof 1 mm or greater and less than 5 mm is referred to as “zone A”,whereas a zone away from the central point of core 4 at a distance of 5mm or greater and 10 mm or less is referred to as “zone B”.

At all points Pa included in the zone A, the following mathematicalexpression (I) is satisfied.

Ha2−Ha1<5  (I)

In this mathematical expression (I), Ha1 represents the JIS-C hardnessof the point Pa1. The point Pa1 is located inside the point Pa along theradial direction. The point Pa1 is away from the point Pa at a distanceof 1 mm. In this mathematical expression (I), Ha2 represents the JIS-Chardness of the point Pa2. The point Pa2 is located outside the point Paalong the radial direction. The point Pa2 is away from the point Pa at adistance of 1 mm. The hardness Ha1 and the hardness Ha2 are measured bypushing a JIS-C type hardness scale on a section of the hemisphere,which had been obtained by cutting the center 10. For the measurement,an automated rubber hardness tester (“P1”, trade name, available fromKobunshi Keiki Co., Ltd.) equipped with this hardness scale is used.

The core 4 that satisfies the above mathematical expression (I) isaccompanied by less energy loss upon hitting with a golf club. This core4 can serve in achieving a high resilience of the golf ball 2. The golfball 2 having this core 4 is excellent in the flight performance. Inlight of the flight performance, the difference (Ha2−Ha1) is morepreferably no greater than 4, and particularly preferably no greaterthan 3. The difference (Ha2−Ha1) may be zero.

At any point Pb included in the zone B, the following mathematicalexpression (II) is satisfied.

Hb2−Hb1≧5  (II)

In this mathematical expression (II), Hb1 represents the JIS-C hardnessof the point Pb1. The point Pb1 is located inside the point Pb along theradial direction. The point Pb1 is away from the point Pb at a distanceof 1 mm. In this mathematical expression (II), Hb2 represents the JIS-Chardness of the point Pb2. The point Pb2 is located outside the point Pbalong the radial direction. The point Pb2 is away from the point Pb at adistance of 1 mm. The hardness Hb1 and the hardness Hb2 are measured bypushing a JIS-C type hardness scale on a section of the hemisphere,which had been obtained by cutting the center 10. For the measurement,an automated rubber hardness tester (“P1”, trade name, available fromKobunshi Keiki Co., Ltd.) equipped with this hardness scale is used.

The core 4 that satisfies the above mathematical expression (II)suppresses the spin of the golf ball 2. In this respect, the difference(Hb2−Hb1) is particularly preferably no less than 7. In light of lessenergy loss upon hitting with a golf club, the difference (Hb2−Hb1) ispreferably no greater than 20, and particularly preferably no greaterthan 15.

The proportion of the volume of the core 4 relative to the volume of thephantom sphere of the golf ball 2 is no less than 76%. In other words,this core 4 is large. This core 4 can serve in achieving a superiorresilience performance of the golf ball 2. This core 4 can suppress thespin of the golf ball 2. In these respects, this proportion is morepreferably no less than 78%, and particularly preferably no less than80%. The surface of the phantom sphere corresponds to the surface of thegolf ball 2 assumed as not having the dimples 14.

In light of suppression of the spin, the difference (He−Hb2) between thesurface hardness He of the core 4 and the hardness Hb2 is preferably noless than 10, and particularly preferably no less than 12. In light ofless energy loss, the difference (He−Hb2) is preferably no greater than20.

For the mid layer 6, a resin composition may be suitably used.Illustrative examples of the base polymer of this resin compositioninclude ionomer resins, styrene block-containing thermoplasticelastomers, thermoplastic polyester elastomers, thermoplastic polyamideelastomers and thermoplastic polyolefin elastomers.

Particularly preferable base polymer is an ionomer resin. The ionomerresins are highly elastic. As described later, this golf ball 2 has athin and soft cover 8. Therefore, upon hitting of this golf ball 2 witha driver, the mid layer 6 is greatly deformed. The mid layer 6containing the ionomer resin is responsible for the resilienceperformance achieved upon shots with a driver. An ionomer resin andother resin may be used in combination. When these are used incombination, the percentage of the amount of the ionomer resin relativeto the total amount of the base polymer is preferably no less than 50%by weight, more preferably no less than 70% by weight, and particularlypreferably no less than 85% by weight, in light of the resilienceperformance.

Examples of preferred ionomer resin include binary copolymers formedwith an α-olefin and an α,β-unsaturated carboxylic acid having 3 to 8carbon atoms. Preferable binary copolymer comprises a 80% by weight ormore and 90% by weight or less α-olefin, and a 10% by weight or more and20% by weight or less α,β-unsaturated carboxylic acid. This binarycopolymer provides excellent resilience performance. Examples of otherionomer resin preferred include ternary copolymers formed with anα-olefin, an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms,and an α,β-unsaturated carboxylate ester having 2 to 22 carbon atoms.Preferable ternary copolymer comprises a 70% by weight or more and 85%by weight or less α-olefin, a 5% by weight or more and 30% by weight orless α,β-unsaturated carboxylic acid, and a 1% by weight or more and 25%by weight or less α,β-unsaturated carboxylate ester. This ternarycopolymer provides excellent resilience performance. In the binarycopolymer and ternary copolymer, preferable α-olefin is ethylene andpropylene, and preferable α,β-unsaturated carboxylic acid is acrylicacid and methacrylic acid. Particularly preferred ionomer resin is acopolymer formed with ethylene, and acrylic acid or methacrylic acid.

In the binary copolymer and ternary copolymer, a part of the carboxylgroups may be neutralized with a metal ion. Illustrative examples of themetal ion for use in the neutralization include sodium ion, potassiumion, lithium ion, zinc ion, calcium ion, magnesium ion, aluminum ion andneodymium ion. The neutralization may be carried out with two or morekinds of the metal ions. Particularly suitable metal ion in light of theresilience performance and durability of the golf ball 2 is sodium ion,zinc ion, lithium ion and magnesium ion.

Specific examples of the ionomer resin include “Himilan® 1555”,“Himilan® 1557”, “Himilan® 1605”, “Himilan® 1706”, “Himilan® 1707”,“Himilan® 1856”, “Himilan® 1855”, “Himilan® AM7311”, “Himilan® AM7315”,“Himilan® AM7317”, “Himilan® AM7318”, “Himilan AM7329”, “Himilan®MK7320” and “Himilan® MK7329”, trade names, available from DuPont-MITSUI POLYCHEMICALS Co., Ltd.; “Surlyn® 6120”, “Surlyn® 6910”,“Surlyn® 7930”, “Surlyn® 7940”, “Surlyn® 8140”, “Surlyn® 8150”, “Surlyn®8940”, “Surlyn® 8945”, “Surlyn® 9120”, “Surlyn® 9150”, “Surlyn® 9910”,“Surlyn® 9945”, “Surlyn® AD8546”, “HPF 1000” and “HPF 2000”, tradenames, available from Du Pont Kabushiki Kaisha; and “IOTEK 7010”, “IOTEK7030”, “IOTEK 7510”, “IOTEK 7520”, “IOTEK 8000” and “IOTEK 8030”, tradenames, available from EXXON Mobil Chemical Corporation.

Two or more kinds of the ionomer resins may be used in combination inthe mid layer 6. An ionomer resin neutralized with a monovalent metalion and an ionomer resin neutralized with a bivalent metal ion may beused in combination.

The mid layer 6 may also contain a highly elastic resin. Illustrativeexamples of the highly elastic resin include polybutylene terephthalate,polyphenylene ether, polyethylene terephthalate, polysulfone, polyethersulfone, polyphenylene sulfide, polyarylate, polyamideimide, polyetherimide, polyether ether ketone, polyimide, polytetrafluoroethylene,polyaminobismaleimide, polybisamide triazole, polyphenyleneoxide,polyacetal, polycarbonate, acrylonitrile-butadiene-styrene copolymersand acrylonitrile-styrene copolymers.

Into the mid layer 6 may be blended a coloring agent such as titaniumdioxide, a filler such as barium sulfate, a dispersant, an antioxidant,an ultraviolet absorbent, a light stabilizer, a fluorescent agent, afluorescent brightening agent and the like in an appropriate amount asneeded. For forming the mid layer 6, a known procedure such as injectionmolding, compression molding and the like may be employed.

The mid layer 6 has the hardness Hm of preferably no less than 90. Themid layer 6 having the hardness Hm of no less than 90 can serve inachieving excellent resilience performance of the golf ball 2. Inaddition, with the mid layer 6 having the hardness Hm of no less than90, an outer-hard/inner-soft structure of a sphere composed of the core4 and the mid layer 6 can be attained. The sphere having anouter-hard/inner-soft structure suppresses the spin of the golf ball 2.In these respects, the hardness Hm is particularly preferably no lessthan 92. In light of the feel at impact, the hardness Hm is preferablyno greater than 98, and particularly preferably no greater than 97. Inlight of suppression of the spin, it is preferred that the hardness Hmof the mid layer 6 is greater than the surface hardness He of the core4, and that the surface hardness He of the core 4 is greater than thesurface hardness of the center 10.

The hardness Hm is measured with a JIS-C type spring hardness scaleattached to an Auto Loading Durometer (automated rubber hardness tester,Kobunshi Keiki Co., Ltd., trade name “P1”). For the measurement, a slabformed by hot press is used. The slab has a thickness of about 2 mm. Theslab which had been stored at a temperature of 23° C. for two weeks isused for the measurement. When the measurement is carried out, threeslabs are overlaid. The slab constituted with the same resin compositionas that of the mid layer 6 is used for the measurement.

In light of suppression of the spin, the thickness of the mid layer 6 ispreferably no less than 0.3 mm, more preferably no less than 0.5 mm, andparticularly preferably no less than 0.6 mm. In light of the feel atimpact, the thickness is preferably no greater than 1.5 mm, morepreferably no greater than 1.2 mm, and particularly preferably nogreater than 1.0 mm.

The cover 8 is constituted with a resin composition. Illustrativeexamples of the base polymer of this resin composition includepolyurethanes, polyesters, polyamides, polyolefins, polystyrenes andionomer resins. In particular, a polyurethane is preferred. Apolyurethane is soft. When the golf ball 2 having a cover 8 in which apolyurethane is used is hit with a short iron, a great spin rate isattained. The cover 8 constituted with a polyurethane is responsible forthe control performance upon shots with a short iron. The polyurethaneis also responsible for the scuff resistance performance of the cover 8.

When this golf ball 2 is hit with a driver, long iron or middle iron,the sphere composed of the core 4 and the mid layer 6 is greatlydistorted due to a high head speed. Since this sphere has anouter-hard/inner-soft structure, the spin rate is suppressed. Due tosuppression of the spin rate, a great flight distance is attained. Whenthis golf ball 2 is hit with a short iron, less distortion of the sphereoccurs since the head speed is low. Behavior of the golf ball 2 uponhitting with a short iron predominantly varies depending on the cover 8.Since the cover 8 containing the polyurethane is soft, a great spin rateis attained. By the great spin rate, an excellent control performance isachieved. According to this golf ball 2, flight performances achievedupon shots with a driver, a long iron and a middle iron, and controlperformances achieved upon shots with a short iron are both achievedwith favorable balance.

When this golf ball 2 is hit, the cover 8 including a polyurethaneabsorbs impact. This absorption leads to a soft feel at impact achieved.In particular, when hit with a short iron or a putter, the cover 8 leadsto an excellent feel at impact achieved.

Into the cover 8, the polyurethane and other resin may be used incombination. When thus used in combination, the polyurethane is includedas a principal component of the base polymer in light of the spinperformance and the feel at impact. The percentage of the amount of thepolyurethane relative to the total amount of the base polymer ispreferably no less than 50% by weight, more preferably no less than 70%by weight, and particularly preferably no less than 85% by weight.

A thermoplastic polyurethane and a thermosetting polyurethane may beused in the cover 8. In light of the productivity, a thermoplasticpolyurethane is preferred. The thermoplastic polyurethane includes apolyurethane component as a hard segment, and a polyester component or apolyether component as a soft segment.

The polyurethane contains a polyol component. As the polyol, a polymerpolyol is preferred. Specific examples of the polymer polyol include:polyether polyols such as polyoxyethylene glycol (PEG), polyoxypropyleneglycol (PPG) and polytetramethylene ether glycol (PTMG); condensedpolyester polyols such as polyethylene adipate (PEA), polybutyleneadipate (PBA) and polyhexamethylene adipate (PHMA); lactone basedpolyester polyols such as poly-ε-caprolactone (PCL); polycarbonatepolyols such as polyhexamethylene carbonate; and acrylic polyols. Two ormore kinds of the polyol may be used in combination.

Particularly, a polytetramethylene ether glycol is preferred. A spinrate attained upon hitting of the golf ball 2 with a short iron has agreat correlation with the content of the polytetramethylene etherglycol. On the other hand, a spin rate attained upon hitting of the golfball 2 with a driver has a less correlation with the content of thepolytetramethylene ether glycol. The golf ball 2 in which thepolyurethane contains an appropriate amount of a polytetramethyleneether glycol is excellent in both terms of the flight performanceachieved upon hitting with a driver, and the control performanceachieved upon hitting with a short iron.

In light of the control performance, the polyol has a number averagemolecular weight of preferably no less than 200, more preferably no lessthan 400, and particularly preferably no less than 650. In light ofsuppression of the spin, the molecular weight is preferably no greaterthan 1,500, more preferably no greater than 1,200, and particularlypreferably no greater than 850.

The number average molecular weight is measured with a gel permeationchromatography. The measurement conditions are as in the following.

Apparatus: HLC-8120GPC (Tosoh Corporation)

Eluent: tetrahydrofuran

Concentration: 0.2% by weight

Temperature: 40° C.

Column: TSKgel Super HM-M (Tosoh Corporation)

Amount of sample: 5 microliter

Flow rate: 0.5 ml/min

Standard substance: polystyrene (Tosoh Corporation, “PStQuick Kit-H”)

Examples of the isocyanate component in the polyurethane include:aromatic polyisocyanates such as 2,4-toluene diisocyanate, 2,6-toluenediisocyanate, mixtures of 2,4-toluene diisocyanate and 2,6-toluenediisocyanate (TDI), 4,4′-diphenylmethane diisocyanate (MDI),1,5-naphthylene diisocyanate (NDI), 3,3′-bitolylene-4,4′-diisocyanate(TODI), xylylene diisocyanate (XDI), tetramethylxylylene diisocyanate(TMXDI) and paraphenylene diisocyanate (PPDI); alicyclic polyisocyanatessuch as 4,4′-dicyclohexylmethane diisocyanate (H₁₂MDI), hydrogenatedxylylene diisocyanate (H₆XDI) and isophorone diisocyanate (IPDI); andaliphatic polyisocyanates such as hexamethylene diisocyanate (HDI). Twoor more polyisocyanates may be used in combination. In light of theweather resistance, TMXDI, XDI, HDI, H₆XDI, IPDI and H₁₂MDI arepreferred.

The polyurethane may contain a chain extender as a component thereof.Illustrative examples of the chain extender include low molecular weightpolyols and low molecular weight polyamines.

The low molecular weight polyols are exemplified by diols, triols,tetraols and hexaols. Specific examples of the diol include ethyleneglycol, diethylene glycol, propane diol, dipropylene glycol, butanediol,neopentyl glycol, pentanediol, hexanediol, heptanediol and octanediol.Specific examples of the triol include glycerin, trimethylolpropane andhexanetriol. Specific examples of the tetraol include pentaerythritoland sorbitol.

The low molecular weight polyamines are exemplified by aliphaticpolyamines, monocyclic aromatic polyamines and polycyclic aromaticpolyamines. Specific examples of the aliphatic polyamine includeethylene diamine, propylene diamine, butylene diamine and hexamethylenediamine. Specific examples of the monocyclic aromatic polyamine includephenylene diamine, toluene diamine, dimethyltoluene diamine,dimethylthiotoluene diamine and xylylene diamine.

The cover 8 may be molded from a composition containing a thermoplasticpolyurethane and an isocyanate compound. During or following molding ofthe cover 8, the polyurethane is crosslinked by this isocyanatecompound.

Into the cover 8 may be blended a coloring agent such as titaniumdioxide, a filler such as barium sulfate, a dispersant, an antioxidant,an ultraviolet absorbent, a light stabilizer, a fluorescent agent, afluorescent brightening agent and the like in an appropriate amount asneeded.

The JIS-C hardness Hc of the cover 8 is no greater than 65. By employingsuch a soft cover 8, a favorable control performance upon shots with ashort iron can be achieved. In light of the control performance, thehardness Hc is more preferably no greater than 60, still more preferablyno greater than 55, and particularly preferably no greater than 50. Whenthe hardness is too low, the flight performance achieved upon shots witha driver may be insufficient. In this respect, the hardness ispreferably no less than 20, more preferably no less than 25, andparticularly preferably no less than 35. For the measurement of thehardness Hc, a slab constituted with the same resin composition as theresin composition of the cover 8 is used. The measuring method issimilar to the measuring method of the hardness Hm of the mid layer 6.

The hardness Hc of the cover 8 is less than the central hardness Ho ofthe core 4. This golf ball 2 is excellent in the control performanceachieved upon shots with a short iron. In light of the controlperformance, the difference (Ho−Hc) is preferably no less than 3, morepreferably no less than 5, and particularly preferably no less than 8.The difference (Ho−Hc) is preferably no greater than 15.

In light of the flight performance achieved upon shots with a driver,the cover 8 has a thickness of preferably no greater than 0.8 mm, morepreferably no greater than 0.6 mm, still more preferably no greater than0.5 mm, and particularly preferably no greater than 0.4 mm. In light ofthe control performance achieved upon shots with a short iron, thethickness is preferably no less than 0.10 mm, and particularlypreferably no less than 0.15 mm.

For forming the cover 8, a known procedure may be employed such asinjection molding, compression molding or the like. Dimples 14 areformed by way of pimples formed on the cavity face of the mold when thecover 8 is molded.

In light of the feel at impact, the amount of compressive deformation Dbof the golf ball 2 is preferably no less than 2.0 mm, more preferably noless than 2.1 mm, and particularly preferably no less than 2.2 mm. Inlight of the resilience performance, the amount of compressivedeformation Db is preferably no greater than 3.5 mm, more preferably nogreater than 3.0 mm, and particularly preferably no greater than 2.6 mm.

Upon measurement of the amount of compressive deformation Db, the golfball 2 is placed on a hard plate made of metal. A cylinder made of metalgradually descends toward this golf ball 2. The golf ball 2 interposedbetween the bottom face of the cylinder and the hard plate is deformed.A migration distance of the cylinder, starting from the state in whichan initial load of 98 N is applied to the golf ball 2 up to the state inwhich a final load of 1,274 N is applied thereto is measured.

The golf ball 2 may have a reinforcing layer between the mid layer 6 andthe cover 8. The reinforcing layer firmly adheres to the mid layer 6,and firmly adheres also to the cover 8. Due to the reinforcing layer,detachment of the cover 8 from the mid layer 6 can be suppressed. Asdescribed above, this golf ball 2 has thin cover 8. When this golf ball2 is hit with an edge of a clubface, a wrinkle is liable to begenerated. The reinforcing layer suppresses generation of such awrinkle.

For the base polymer of the reinforcing layer, a two-component curedthermosetting resin may be suitably used. Specific examples of thetwo-component cured thermosetting resin include epoxy resins, urethaneresins, acrylic resins, polyester based resins and cellulose basedresins. In light of the strength and durability of the reinforcinglayer, two-component cured epoxy resins and two-component cured urethaneresins are preferred.

The reinforcing layer may include additives such as a coloring agent(typically, titanium dioxide), a phosphate based stabilizer, anantioxidant, a light stabilizer, a fluorescent brightening agent, anultraviolet absorbent, a blocking preventive agent and the like. Theadditive may be added either to the base material of the two-componentcured thermosetting resin, or to the curing agent of the two-componentcured thermosetting resin.

The reinforcing layer is obtained by coating a liquid, which had beenprepared by dissolving or dispersing a base material and a curing agentin a solvent, on the surface of the mid layer 6. In light of theworkability, coating with a spray gun is preferred. The solvent isvolatilized after the coating to permit a reaction of the base materialwith the curing agent, thereby forming the reinforcing layer.

In light of suppression of the wrinkle, the reinforcing layer has athickness of preferably no less than 3 μm, and more preferably no lessthan 5 μm. In light of ease of forming the reinforcement layer, thethickness is preferably no greater than 300 μm, more preferably nogreater than 50 μm, and particularly preferably no greater than 20 μm.The thickness is measured by observation of the cross section of thegolf ball 2 with a microscope. When the surface of the mid layer 6 hasroughness resulting from a surface roughening treatment, the thicknessis measured immediately above the protruded portion.

In light of suppression of the wrinkle, the reinforcing layer has apencil hardness of preferably no less than 4B, and more preferably noless than B. In light of less loss of the force during transfer from thecover 8 to the mid layer 6 upon hitting of the golf ball 2, thereinforcing layer has a pencil hardness of preferably no greater than3H. The pencil hardness is measured in accordance with a standard of“JIS K5400”.

EXAMPLES Example 1

A rubber composition (1) was obtained by kneading 100 parts by weight ofa high-cis polybutadiene (“BR-730”, trade name, available from JSRCorporation), 20 parts by weight of zinc diacrylate, 5 parts by weightof zinc oxide, an adequate amount of barium sulfate, 0.5 part by weightof diphenyl disulfide and 0.7 part by weight of dicumyl peroxide. Thisrubber composition (1) was placed into a mold having upper and lowermold half each having a hemispherical cavity, and heated at atemperature of 170° C. for 15 minutes to obtain a center having adiameter of 15 mm.

A rubber composition (3) was obtained by kneading 100 parts by weight ofa high-cis polybutadiene (“BR-730”, supra), 42 parts by weight of zincdiacrylate, 5 parts by weight of zinc oxide, an adequate amount ofbarium sulfate, 0.5 part by weight of diphenyl disulfide and 0.7 part byweight of dicumyl peroxide. Half shells were formed from this rubbercomposition (3). The aforementioned center was covered by two pieces ofthe half shell. The center and the half shells were placed into a moldhaving upper and lower mold half each having a hemispherical cavity, andheated at a temperature of 170° C. for 20 min to obtain a core having adiameter of 39.7 mm. An envelope layer was formed from the rubbercomposition (3). The amount of barium sulfate was adjusted such that theenvelope layer has a specific gravity identical to the specific gravityof the center, and the ball has a weight of 45.4 g.

A resin composition (a) was obtained by kneading 50 parts by weight ofan ionomer resin (“Surlyn® 8945”, supra), and 50 parts by weight ofother ionomer resin (“Himilan® AM7329”, supra) in a biaxial kneadingextruder. The core was placed into a mold having upper and lower moldhalf each having a hemispherical cavity. The resin composition (a) wasinjected around the core by injection molding, whereby a mid layer wasformed. This mid layer had a thickness of 1.0 mm.

A paint composition containing a two-component cured epoxy resin as abase polymer (“POLIN 750LE”, trade name, available from Shinto PaintCo., Ltd.) was prepared. The base material liquid of this paintcomposition consists of 30 parts by weight of a bisphenol A type solidepoxy resin and 70 parts by weight of a solvent. The curing agent liquidof this paint composition consists of 40 parts by weight of denaturedpolyamide amine, 55 parts by weight of a solvent and 5 parts by weightof titanium dioxide. The weight ratio of the base material liquid andthe curing agent liquid was 1/1. This paint composition was coated onthe surface of the mid layer with a spray gun, and kept in an atmosphereof 40° C. for 24 hrs to give a reinforcing layer. This reinforcing layerhad a thickness of 10 μm.

A resin composition (b) was obtained by kneading 100 parts by weight ofa thermoplastic polyurethane elastomer (“Elastollan® XNY85A”, tradename, available from BASF Japan Ltd.) and 4 parts by weight of titaniumdioxide in a biaxial kneading extruder. Half shells were obtained fromthis resin composition (b) with compression molding. A sphere composedof the core, the mid layer and the reinforcing layer was covered by twopieces of the half shell. The sphere and half shells were placed into afinal mold having upper and lower mold half each having a hemisphericalcavity and being provided with a large number of pimples on the cavityface thereof. A cover was obtained by compression molding. This coverhad a thickness of 0.5 mm. Dimples having a shape inverted from theshape of the pimple were formed on the cover. A clear paint including atwo-component cured polyurethane as a base material was applied on thiscover to give a golf ball of Example 1 having a diameter of 42.7 mm. Thehardness distribution of the core of this golf ball is shown in Table 3.

Examples 2 to 8 and Comparative Examples 1 to 4

Golf balls of Examples 2 to 8, and Comparative Examples 1 to 4 wereobtained in a similar manner to Example 1 except that specifications ofthe center, the envelope layer, the mid layer and the cover were aslisted in Tables 6 to 8 below. Details of the rubber compositions of thecore are presented in Table 1 below. Details of the resin compositionsof the mid layer and the cover are presented in Table 2 below. Thehardness distribution of the core is shown in Tables 3 to 6. The golfball according to Comparative Example 2 does not have an envelope layer.

[Shot with Driver (W#1)]

A driver with a titanium head (SRI Sports Limited, trade name “SRIXONW505”, shaft hardness: X, loft angle: 8.5°) was attached to a swingmachine available from Golf Laboratory Co. Then the golf ball was hitunder a condition to give the head speed of 50 m/sec. The ball speed andspin rate immediately after the hitting, and the distance from thelaunching point to the point where the ball stopped were measured. Meanvalues of the data obtained by measuring 12 times are shown in Tables 6to 8 below.

[Shot with Short Iron]

A sand wedge (SW) was attached to a swing machine available from GolfLaboratory Co. Then the golf ball was hit under a condition to give thehead speed of 21 m/sec, and the spin rate immediately after the hittingwas measured. Mean values of the data obtained by measuring 12 times areshown in Tables 6 to 8 below.

[Feel at Impact]

The golf balls were hit by ten golf players with a sand wedge, and aninterview was conducted on the feel at impact. Based on the number ofgolf players who evaluated that “the feel at impact was favorable”,rating was performed according to the following criteria.

A: 8 or more

B: 6-7

C: 4-5

D: 3 or fewer

The results are shown in the following Tables 6 to 8.

TABLE 1 Composition of Core (part by weight) (1) (2) (3) (4) (5) BR-730100 100 100 100 100 Zinc diacrylate 20 38 42 45 39 Zinc oxide 5 5 5 5 5Barium sulfate * * * * * Diphenyl disulfide 0.5 0.5 0.5 0.5 0.5 Dicumylperoxide 0.7 0.7 0.7 0.7 0.7 * Adequate amount

TABLE 2 Composition of Mid Layer and Cover (part by weight) (a) (b) (c)(d) (e) (f) (g) (h) Surlyn ® 8945 50 — — — — — — — Himilan ® 50 — — — —— — — AM7329 Elastollan ® — 100 — — — — — — XNY85A Elastollan ® — — 100— — — — — XNY90A Elastollan ® — — — 100 — — — — XNY97A Polyurethane *1 —— — — 100 — — — Polyurethane *2 — — — — — 100 — — Polyurethane *3 — — —— — — 100 — Polyurethane *4 — — — — — — — 100 Titanium dioxide — 4 4 4 44 4 4 Hardness (JIS-C) 94 47 56 67 45 42 42 38 Hardness 64 32 38 47 3028 28 25 (Shore D)Any of Elastollan® XNY85A, Elastollan® XNY90A, Elastollan® XNY97A,polyurethane *1, polyurethane *2, polyurethane *3 and polyurethane *4 isa thermoplastic polyurethane elastomer including a polytetramethyleneether glycol as a polyol component. The number average molecular weightof the polytetramethylene ether glycol is as in the following.

Elastollan® XNY85A: 1,800

Elastollan® XNY90A: 1,800

Elastollan® XNY97A: 1,800

polyurethane *1: 1,500

polyurethane *2: 1,000

polyurethane *3: 850

polyurethane *4: 650

TABLE 3 Hardness Distribution of Core (JIS-C) Distance from the centralpoint (mm) Example 1 Example 2 Example 3 Example 4 0 60 60 60 60 1.0 6160.8 61 61 2.0 62 61.6 62 62 3.0 63 62.4 63 63 4.0 64 63.2 64 64 5.0 6564 65 65 6.0 66 65 66 66 7.0 67 66 66 67 8.0 75 67 75 75 9.0 76 — 76 7610.0 77 77 77 77 11.0 78 78 78 78

TABLE 4 Hardness Distribution of Core (JIS-C) Distance from the centralpoint (mm) Example 5 Example 6 Example 7 0 60 60 60 1.0 61 61 61 2.0 6262 62 3.0 63 63 63 4.0 64 64 64 5.0 65 65 65 6.0 66 66 66 7.0 67 67 678.0 75 75 75 9.0 76 76 76 10.0 77 77 77 11.0 78 78 78

TABLE 5 Hardness Distribution of Core (JIS-C) Distance from the Compara.Compara. Compara. Compara. central point (mm) Example 1 Example 2Example 3 Example 4 0 60 70 60 60 1.0 61 71.2 60.9 61 2.0 62 72.4 61.862 3.0 63 73.6 62.7 63 4.0 64 74.8 63.6 64 5.0 65 76 64.5 65 6.0 66 7665.4 65.5 7.0 67 76 66.3 66 8.0 75 76 67.2 73 9.0 76 76 68.1 74 10.0 7776 69 75 11.0 78 77 70 76

TABLE 6 Evaluation Results Example 1 Exampl 2 Example 3 Example 4 CenterComposition (1) (1) (1) (1) Crosslinking 170 170 170 170 temperature (°C.) Crosslinking time (min) 15 15 15 15 Diameter (mm) 15 18 15 15Envelope layer Composition (3) (3) (3) (3) Crosslinking 170 170 170 170temperature (° C.) Crosslinking time (min) 20 20 20 20 Core Diameter(mm) 39.7 40.1 40.3 39.7 Volume proportion (%) 80.4 82.8 84.1 80.4Hardness Ho (JIS-C) 60 60 60 60 Hardness He (JIS-C) 88 88 88 88 Midlayer Composition (a) (a) (a) (a) Hardness (JIS-C) 94 94 94 94 Thickness(mm) 1.0 1.0 0.9 1.0 Cover Composition (b) (c) (e) (e) Hardness (JIS-C)47 56 45 45 Thickness (mm) 0.5 0.3 0.3 0.5 Ball Deformation Db (mm) 2.402.45 2.40 2.40 Ha2 − Ha1 (maximum value) 2 1.6 2 2 Hb2 − Hb1 (maximumvalue) 9 10 10 9 W #1 Ball speed (m/s) 73.9 74.0 74.1 73.9 Spin (rpm)2,440 2,310 2,410 2,370 Flight distance (m) 248.5 251.0 250.0 249.5 SWSpin (rpm) 6,720 6,530 6,690 6,710 Feel at impact A B A A

TABLE 7 Evaluation Results Example 5 Example 6 Example 7 CenterComposition (1) (1) (1) Crosslinking 170 170 170 temperature (° C.)Crosslinking time (min) 15 15 15 Diameter (mm) 15 15 15 EnvelopeComposition (3) (3) (3) layer Crosslinking 170 170 170 temperature (°C.) Crosslinking time (min) 20 20 20 Core Diameter (mm) 39.7 39.7 39.7Volume proportion (%) 80.4 80.4 80.4 Hardness Ho (JIS-C) 60 60 60Hardness He (JIS-C) 88 88 88 Mid Composition (a) (a) (a) layer Hardness(JIS-C) 94 94 94 Thickness (mm) 1.0 1.0 1.0 Cover Composition (f) (g)(h) Hardness (JIS-C) 42 42 38 Thickness (mm) 0.5 0.5 0.5 BallDeformation Db (mm) 2.40 2.40 2.40 Ha2 − Ha1 (maximum value) 2 2 2 Hb2 −Hb1 (maximum value) 9 9 9 W #1 Ball speed (m/s) 73.9 73.9 73.9 Spin(rpm) 2,350 2,320 2,370 Flight distance (m) 250.0 250.5 249.5 SW Spin(rpm) 6,690 6,640 6,770 Feel at impact A A A

TABLE 8 Evaluation Results Compa. Compa. Compa. Compa. Example 1 Example2 Example 3 Example 4 Center Composition (1) (2) (1) (1) Crosslinking170 170 170 170 temperature (° C.) Crosslinking time (min) 15 20 15 15Diameter (mm) 15 39.7 25 15 Envelope layer Composition (3) — (4) (5)Crosslinking 170 — 170 170 temperature (° C.) Crosslinking time (min) 20— 20 20 Core Diameter (mm) 39.7 39.7 39.1 38.5 Volume proportion (%)80.4 80.4 76.8 73.3 Hardness Ho (JIS-C) 60 70 60 60 Hardness He (JIS-C)88 86 90 86 Mid layer Composition (a) (a) (a) (a) Hardness (JIS-C) 94 9494 94 Thickness (mm) 1.0 1.0 1.0 1.6 Cover Composition (d) (b) (b) (b)Hardness (JIS-C) 67 47 47 47 Thickness (mm) 0.5 0.5 0.8 0.5 BallDeformation Db (mm) 2.40 2.40 2.40 2.40 Ha2 − Ha1 (maximum value) 2 2.41.8 2 Hb2 − Hb1 (maximum value) 9 1 1.9 8 W #1 Ball speed (m/s) 74.074.0 73.5 73.6 Spin (rpm) 2,270 2,580 2,360 2,420 Flight distance (m)251.5 247.0 246.0 247.0 SW Spin (rpm) 6,400 6,750 6,670 6,610 Feel atimpact B A A B

As is shown in Tables 6 to 8, the golf balls according to Examples areexcellent in various performances. Therefore, advantages of the presentinvention are clearly suggested by these results of evaluation.

The golf ball according to the present invention can be used for theplay at the golf course, and the practice at the driving range. Theforegoing description is just for illustrative examples; therefore,various modifications can be made in the scope without departing fromthe principles of the present invention.

1. A golf ball comprising a core, a mid layer situated on the externalside of the core, and a cover situated on the external side of the midlayer, wherein: the proportion of the volume of the core relative to thevolume of the phantom sphere of the golf ball is no less than 76%; theJIS-C hardness Hc of the cover is less than the JIS-C hardness Ho of thecentral point of the core; at all points Pa included in zone “A” awayfrom the central point of the core at a distance of 1 mm or greater andless than 5 mm, the following mathematical expression (I) is satisfied;and at any point Pb included in zone “B” away from the central point ofthe core at a distance of 5 mm or greater and 10 mm or less, thefollowing mathematical expression (II) is satisfied:Ha2−Ha1<5  (I); andHb2−Hb1≧5  (II), in the above mathematical expression (I), Ha1represents a JIS-C hardness at a point Pa1 that is located inside thepoint Pa along the radial direction and away from the point Pa at adistance of 1 mm, and Ha2 represents the JIS-C hardness at a point Pa2that is located outside the point Pa along the radial direction and awayfrom the point Pa at a distance of 1 mm; and in the above mathematicalexpression (II), Hb1 represents the JIS-C hardness at a point Pb1 thatis located inside the point Pb along the radial direction and away fromthe point Pb at a distance of 1 mm, and Hb2 represents a JIS-C hardnessat a point Pb2 that is located outside the point Pb along the radialdirection and away from the point Pb at a distance of 1 mm.
 2. The golfball according to claim 1, wherein the JIS-C hardness Hc of the cover isno greater than
 65. 3. The golf ball according to claim 1, wherein thecover has a thickness of no greater than 0.8 mm.
 4. The golf ballaccording to claim 1, wherein: a principal component of the basematerial of the cover is a thermoplastic polyurethane; and a polyolcomponent of the thermoplastic polyurethane is a polytetramethyleneether glycol having a number average molecular weight of no greater than1,500.
 5. The golf ball according to claim 1, wherein the JIS-C hardnessHm of the mid layer is no less than
 90. 6. The golf ball according toclaim 1, wherein the mid layer has a thickness of no greater than 1.5mm.
 7. The golf ball according to claim 1, wherein the differencebetween the JIS-C hardness He of the surface of the core and thehardness Hb2 is no less than
 10. 8. The golf ball according to claim 1,wherein the difference between the JIS-C hardness He of the surface ofthe core and the hardness Ho is no greater than
 40. 9. The golf ballaccording to claim 1, wherein the difference between the hardness Ho andthe hardness Hc is 3 or greater and 15 or less.
 10. The golf ballaccording to claim 1, wherein the hardness Ho is 40 or greater and 80 orless.
 11. The golf ball according to claim 1, wherein the JIS-C hardnessHe of the surface of the core is 75 or greater and 95 or less.
 12. Thegolf ball according to claim 1, wherein the JIS-C hardness Hm of the midlayer is greater than the JIS-C hardness He of the surface of the core.13. The golf ball according to claim 1, wherein the core has a center,and an envelope layer situated on the external side of the center. 14.The golf ball according to claim 13, wherein the center has a diameterof 10 mm or greater and 20 mm or less.
 15. The golf ball according toclaim 13, wherein the envelope layer has a thickness of 8 mm or greaterand 18 mm or less.
 16. The golf ball according to claim 13, wherein thedifference between the JIS-C hardness He of the surface of the envelopelayer and the JIS-C hardness Hi of the innermost point of the envelopelayer is 10 or greater and 25 or less.
 17. The golf ball according toclaim 13, wherein the JIS-C hardness He of the surface of the core isgreater than the JIS-C hardness of the surface of the center.